The present invention relates to a receiver hardly affected by interference and jamming with a simple construction and a rapid synchronization acquisition (search) in a spread spectrum communication system, particularly in a frequency hopping communication system.
The spread spectrum (SS) communication system has been used in a variety of fields. In the communication system of this type, the transmitted bandwidth is much greater than the bandwidth or rate of the information to be transmitted. The carrier wave is modulated by some other function to widen or spread the bandwidth for transmission. The received signal is remapped into the original information bandwidth to reproduce a desired signal. The SS communication system has many useful adavantages: a selective call is possible; since the power spectrum density is low, private communication is allowed; it is little influenced by interference. From this standpoint, the SS system has found many uses, such as mobile communications, avionics systems, satellite communications, direction finders and distance measuring equipment.
The SS system is categorized into a direct sequence (DS) system, a frequency hopping (FH) system, a time hopping (TH) system and a hybrid system which is a proper combination of these systems as just mentioned. Of those communications systems, the FH system is frequently used in the field of mobile communication system with a low traffic volumn for a number of stations.
In the FH system a carrier frequency is shifted or jumped in discrete increments in a pattern dictated by prepared code sequences (e.g. PN code, M-sequence codes, Gold codes and the like) in synchronism with a change in a state of the codes. The resulting consecutive and time sequential frequency pattern is called a hopping pattern and the duration of each hopping frequency is called a chip. The transmitting information is embedded in the codes or embedded in each frequency of the carrier wave by a so-called FSK (frequency shift keying) modulation. The information signal thus spread-spectrum-modulated is reproduced by a receiver.
In reproducing the information signal by the receiver, a synchronization (sync) acquisition process is first performed, in which the code pattern provided in the receiver is made accurately coincident with the code pattern generated in the transmitter in time-position. Then, the SS signal is despread, and thereafter a well-known demodulation is performed to extract the desired information. More particularly, a local reference signal of a frequency correspondingly determined by the same code pattern as that in the transmitter for every chip and the received signal are mixed in a mixer in order to perform a correlation (despreading) process for converting the SS signal into the signal having a frequency bandwidth wide enough to extract the information. This system is described in detail in "SPREAD SPECTRUM SYSTEMS" by R. C. Dixon, published by John Wiley & Sons ZInc. in 1976. Following this despreading process, the desired information is extracted by a usual demodulation technique.
In the FH system, the number of the frequencies to be selected generally reaches several thousands and the carrier frequency changes over a wide frequency bandwidth irregularly. For this reason, a broad band filter for suppressing signals existing outside the SS frequency bandwidth is provided preceding a sync section and a demodulating section. In this case, a signal in the SS frequency bandwidth coming from another station adjacent to that of the station under discussion is directly applied to the mixer through the broad band filter. If the level of the signal transmitted from the adjacent station is extremely high, the mixer operates outside its linear dynamic characteristic, so that it produces higher harmonic components to cause problems of jamming, interference, etc. For example, a ring modulation circuit using diodes has usually been employed for the mixer. When an input signal with a level exceeding the linear dynamic range of the diode comes in, a higher harmonic component is contained in its output signal. As a result, a number of unnecessary signals (spurious signals) are supplied into the circuits succeeding to the mixer. This causes an erroneous sync acquisition, data error, etc, to adversely affect a normal communication.
The signal, after passing through the broad band-pass filter, enters the demodulating section and the sync section. The sync process in the sync section contains a sync acquisition process for timing a local reference signal generated according to the code pattern in the receiver with the received signal and a sync tracking process for keeping the sync state. The sync tracking is performed by the known DLL (delay line lock) circuit. For the details of the sync tracking, reference is made to the above "SPREAD SPECTRUM SYSTEMS" by R. C. Dixon. In the specification, however, no further explanation of the sync tracking will be given, since the sync tracking process is not directly pertinent to the present invention.
Sync acquisition systems now proposed are categorized into active correlation systems and a passive correlation systems, as discussed by M. Pandit in his paper "Mean acquisition time of active- and passive-correlation acquisition systems for spread-spectrum communication systems", IEE PROC., Vol. 128, Pt. F, No. 4, AUGUST 1981; PP 211 to 214. A typical example of the former correlation system is a sliding correlation system described in "6.1 INITIAL SYNCHRONIZATION" on page 180 and the subsequent pages in the above text "SPREAD SPECTRUM SYSTEMS". In the sliding correlation system, a time relation between the received signal and the local reference signal is discretely slid by changing a phase of a clock signal for driving the local reference signal and a correlation between the received signal and the local reference signal is observed. The phase changing of the clock is stopped at a maximum point of the correlation. The sliding correlation system is advantageous in that the construction is simple, but disadvantageous in long acquisition time for sync because of discrete change of the clock phase.
A typical example of the latter correlation system capable of a short acquisition, or the passive correlation system, is a correlation system using a matched filter. In this system, selective receiving channels are specially provided for receiving certain predetermined information. At the time that the information are received by all of the receiving channels, the correlation system judges that the sync is acquired. However, for applying this correlation system to a frequency hopping spread spectrum (FH-SS) communication system, it is necessary to use the selective receiving channels (each containing a frequency synthesizer, a correlator, an IF amplifier, an envelope detector, etc.) equal in number to the hopping frequencies. Further, a number of delay lines for timing the output signals from the receiving channels are required. This complicates the construction of the correlation system.
As described above, various systems as the sync acquisition means for the SS communication system have been proposed. So far as we know, however, there has never been realized a practical sync acquisition means appropriately adaptable for the FH-SS communication SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a receiver in a frequency hopping communication system which is hardly affected by interference and jamming, and which has a simple construction.
Another object of the present invention is to provide a receiver in a frequency hopping communication system having a simplified synchronizing section with a short synchronization acquisition time.
Yet another object of the present invention is to provide a receiver in a frequency hopping communication system having a synchronizing section hard to erroneously operate in response to jamming and interference signals.
A further object of the present invention is to provide a receiver in a frequency hopping communication system having a synchronizing section insensitive to a chip drop in the received signal and capable of a highly accurate synchronization acquisition.
A receiver in a frequency hopping communication system according to the present invention comprises, at a stage prior to the demodulating and synchronizing sections, a filter means including a broad band-pass filter with a band characteristic covering a spread spectrum bandwidth, and a narrow band-pass filter with a plurality of band characteristics obtained by dividing the spread spectrum bandwidth into a plurality of subbands. A band characteristic of the narrow band-pass filter is selected responsive to a selection signal applied thereto and the broad band-pass filter is switched over to the narrow band-pass filter in response to a synchronization detected signal applied thereto. The receiver also has a relatively simple synchronization acquisition means. The synchronization acquisition means employs a relatively small number of receiving channels for selectively receiving signals of predetermined frequencies corresponding to predetermined hopping frequencies. After the output signals from the receiving channels are timed, these signals are summed. When the summed signal exceeds a predetermined threshold value, it is judged that synchronization is achieved and the synchronization detected signal is produced. According to another aspect of the present invention, the sycnchronization acquisition means merely includes a correlator for correlating a local reference signal from the frequency synthesizer and a received signal, a detecting means for envelope-detecting the output signal from the correlator, an integrating means for integrating the output signal from said detecting means to produce an output signal when the integrated value is above a predetermined threshold value, and a gate circuit for producing the synchronization detected signal indicating the set-up of a perfect synchronization when either of the output signals from the detecting means and the integrating means is present.